1. Field of the Invention
The present invention relates to a method for reducing scale during oil and gas production. The present invention particularly relates to non-chemical scale reduction during oil and gas production.
2. Background of the Art
Petroleum fluids primarily comprise oil and water and are herein referred to as formation fluids. A formation fluid may also contain natural gas, and will often contain oil and water insoluble compounds such as clay, silica, waxes, and asphaltenes, which exist as colloidal suspensions.
In addition to the already listed components, formation fluids can also include inorganic components that can precipitate to form mineral scales. The process of mineral scale precipitation is known as scaling. Of primary concern to this invention are mineral scales and scaling. The most common scale forming ions are calcium and barium, but sodium, carbonate, bicarbonate, chloride, sulfate, and strontium are also recognized as scaling species. The most common speciation of these combined scaling ions are: calcium carbonate (CaCO3), calcium sulfate (CaSO4), barium sulfate (BaSO4), and strontium sulfate (SrSO4). In addition, there are less common scale species, such as calcium fluoride (CaF2), iron sulfide (FexSx+1), zinc sulfide (ZnS), lead sulfide (PbS) and sodium chloride (NaCl).
Scale precipitation is primarily affected by commingling of incompatible produced waters and/or changes in physical properties intrinsic to the well system such as: temperature, pressure, fluid turbulence, fluid flow rate, and pH. Specifically, well equipment in positions where incompatible water commingles and/or changes in these intrinsic physical properties occur is particularly vulnerable to scale precipitation. It has also been recognized that well equipment and topside equipment downstream of these sites are also susceptible to scale precipitation in the well system. Any mineral scale sticking to the well system surfaces may narrow pipes, and clog wellbore perforations, various flow valves, and other wellsite and downhole equipment, which results in wellsite equipment failures. It may also slow down, reduce, or even totally prevent the flow of formation fluid into the wellbore and/or out of the wellhead. These effects also extend to crude oil storage facilities that incur maintenance or capacity problems when mineral scale precipitations remain undetected for extended periods of time.
Since mineral scale deposits are undesirable due to these aforementioned problems, it is known in the field of oil and gas production to remove scale from downhole. U.S. Pat. 5,592,243, to Maki, Jr. et al., discloses that an apparatus can be lowered into a borehole that directs high energy sound to the borehole wall and near wellbore formation to dissolve or resuspend material reducing the permeability or the formation or restricting formation fluid flow within the borehole. U.S. Pat. 5,727,628, to Patzner, discloses a similar device that also includes a pump for pumping out water contaminated with the redissolved or resuspended materials.
While the above referenced devices may be effective at cleaning a well that is already afflicted with scale, it is more preferable to avoid scaling altogether, thereby rendering cleaning unnecessary. For example, during oil and gas production in production wells, the drilling of new wells, or workovers of existing wells, many chemicals, referred herein as “additives”, which include scale inhibitors, are often injected from a surface source into the wells to treat the formation fluids flowing through such wells to prevent or control the precipitation of mineral scale. These additives can be injected through a conduit or tubing that is run from the surface to a known depth within the formation and typically upstream of the problem location. In addition, an additive can be injected into a near wellbore formation via a technique commonly referred to as “squeeze” treatment, from which the additive can be slowly released into the formation fluid. Sometimes, additives are introduced in connection with electrical submersible pumps, as shown for example in U.S. Pat. No. 4,582,131, or through an auxiliary line associated with a cable used with the electrical submersible pump, such as is shown in U.S. Pat. No. 5,528,824.
Despite their effectiveness, the use of chemical additives is not without problems. Applying chemical additives efficiently, particularly in remote oil fields that do not permit easy access for chemical delivery and onsite monitoring, is sometimes difficult. Similarly difficult to implement are solutions such as that of WIPO Publication No. WO 00/79095 A1, to Acton, et al., which discloses using ultrasound to create seed crystals for injection into a supersaturated solution to reduce deposition of a mineral salt. The apparatus disclosed is too bulky for use downhole and requires an external power supply. Such an device would be difficult to use in oil field operations, particularly when such operations are in remote locations.